That's a lot to explain for a small character. Hawkeye and Black Widow where kinda just... soldiers and spies. Stuff that has a one word explanation. There is no 'what is a bow, how does he use it so well.' It's all very obvious and intuitive and naturally grounded because it's so real. The others had movies to introduce themselves in. Black Panther is superhuman, but why? He has all this epic technology but why? He has all these uber martial arts... from where? He has this regal attitude and authority... but did he earn that, or is he just a spoiled brat? If this Wakanda has all this vibranium, how come we haven't heard of them before? He's a really, really, bad character to try and represent well in twelve minutes of screen time.

As for diversity... when I google "Avengers diversity" I get a number of results that suggest a vast array of people with a number of different motivations and perspectives have all noticed that all of the Avengers are white. Going a step further, the only female is non-powered, and further, she wasn't even supposed to be there save for Whedon fighting for her to be there. So, yeah, those balances, for various reasons, matter, and are worth resolving. It's different for the JLA that really does have a token female and a token black guy, but the Avengers have a number of non-whites that have historically and popularly been part of the team... expecting them to be included in an awesome vitally important way makes sense.

I was going to respond to him, but you took the words right out of my mouth. With SW and QS already on board, there's no time to include the amount of story you'd need for BP.

Also, yes, the Avengers are hurting for a more diverse squad. In a bad way.